The prior art teaches that information can be transmitted through an information channel by representing the information in terms of lapse time between two successive transitions between electrical states in the system, see U.S. Pat. No. 3,510,780. This type of encoding and decoding of information can be termed as a form of pulse position modulation, also referred to as PPM. In a binary system, as taught by the '780 patent, a first lapsed time, by example one millisecond, may represent a binary zero. A second and greater period of lapsed time, for example a one and one-half millisecond, may represent a binary one. The end of message, by example, being an elapsed time of two milliseconds. This type of scheme not only provides an indication of information, i.e. the binary state, but also provides self clocking information, and may utilize NRZI binary coding, (i.e. wherein a binary zero indicates no change in state, hence no information change, while a binary one indicates a change in state). A distinct advantage results during decoding of data encoded in the foregoing manner in that only pulse/clock generator circuit elements need be employed in the decode circuitry instead of the traditional pulse detector and phase-lock loop circuit elements that must be sensitive to intersymbol interference. Thus, since in a PPM coding scheme the transition delay determines the data value, the intersymbol interference that occurs using traditional peak detector/phase-lock loop circuitry is minimized.
The known encoding circuits and decoding circuits used to implement PPM code schemes, such as taught by U.S. Pat. No. 3,510,780, are limited by the use of encoding and decoding circuit designs that produce binary data transitions whose time delay difference between the ONE state and the ZERO state is 50% or greater.
Another measure of data transmission parameters which relates to data throughput concerns the data recovery resolution factor, i.e. the ratio between the amplitudes of the data signal's highest frequency component and the lowest frequency component. Presently known decoding systems have resolution factors that are above 60% on a normalized data signal amplitude versus frequency curve. The 60% resolution factor is viewed as a data compaction limitation, i.e. the higher the resolution factor, the lower the rate of data transmission. This data compaction limitation of presently known systems being a phenomena related to management of intersymbol interference and flux reversals on the recording medium that impact accurate data bit transition detection.
Thus, a need is seen to exist for a PPM encoding and decoding circuit arrangement which can not only encode and decode a PPM type of code scheme, but which can further produce a constant stream of data transitions having a percentage differences of less than 10% between the time delays which represent the different binary states, and hence which can reliably record and read data at resolution factors of less than sixty percent (60%).
A need is also seen for a data encoding and decoding circuit scheme which can facilitate recovery of data at rates of at least 200 mega/bits per second (MBPS), which rates are not presently achievable in encode and decode circuit schemes for use in local area network (also referred to as LAN) and disk drive computer applications.